Silver halide photographic emulsions have been used for more than a century, and silver halide grains have been the subject of zealous studies for many years. One of the most striking characteristics of silver halide emulsions is their excellent sharpness.
Factors determining the sharpness of a silver halide photographic material obtained by coating silver halide emulsions on a support, and then drying them, are as follows:
(1) Light scattering: Rays of light incident upon a photographic material are scattered by silver halide grains, resulting in lower sharpness.
(2) Granularity: An image obtained after development of a photographic material has a characteristic called granularity, which can be interpreted as a random-dot model and is basically attributed to fluctuations in developing individual silver halide grains.
In T. H. James, The Theory of the Photographic Process, 4th Ed., dependence of the scattering factor on particle size for AgBr grains and AgCl grains in emulsion films are shown in FIG. 20.6 and FIG. 20.7, respectively (on page 582). As is apparent from those figures, the light scattering factor shows a clear dependence on the grain size. More specifically, the light scattering efficiency factor decreases steeply when the grain size becomes extremely small (0.1 .mu. or less).
In the above-cited book, the relationship between the grain size and the granularity are shown in FIG. 21.72, which indicates that the granularity improves with a decrease in grain size. Therefore, it is understandable that the reduction of grain size is very effective for the achievement of high sharpness.
On the other hand, although silver is indispensable for silver halide emulsions, it should be used in the smallest possible amount because of its cost and finiteness as a resource. In general, the transmission density of a developed silver halide emulsion coat is expressed by the following formula (1), called the Nutting equation: EQU D=0.434 na/A (1)
where D is the transmission density, n is the number of grains in an area A, a is the mean projected grain area, and A is the area of the sampling aperture of the densitometer. When the total volume of silver grains present in the area A is taken as M, and the size of an emulsion grain is expressed in terms of a radius (r) of the sphere equivalent in volume, the following relations hold: ##EQU1##
Substituting the above formulae (3) and (4) in the formula (1) yields the following equation (5): EQU D=0.3255 M/(r.multidot.A) (5)
That is, when a particular amount of silver is used, the density obtained (D) is inversely proportional to the grain radius. Accordingly, silver halide grains of smaller size are required to attain a higher transmission density.
In the field of graphic arts, on the other hand, silver halide light-sensitive materials containing water-soluble rhodium salts are disclosed, e.g., in JP-A-60-83083 and JP-A-60-162246 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") with the intention of obtaining a daylight photosensitive material of low sensitivity. However, the addition of rhodium salts in an amount large enough to lower the sensitivity hinders the contrast-increasing effect of hydrazine compounds, resulting in a failure to provide the desired image of sufficiently high contrast.
Because sensitivity is lowered with a decrease in grain size, the diminution in grain size is more desirable for the lowering of sensitivity than the addition of water-soluble rhodium salts. Thus, superfine grains smaller in size are desired.
As for the conventional arts, a "Lippmann" emulsion having an average grain size of 0.050 .mu.m is disclosed as a silver bromide fine grain emulsion, e.g., in T. H. James, The Theory of the Photographic Process, 4th Ed. "Lippmann" emulsions have an average grain size in the range of 0.05 to 0.1 .mu.m, and they are of great importance for photographic plates or films having high resolution, e.g., microphotographs, astrophotographs, masks for production of electronic integrated circuits, holograms, and so on.
Attempts to change operating conditions during the precipitation of silver halides have been made for the purpose of obtaining superfine grains having an average grain size of 0.05 .mu.m or less. In one method, adding an aqueous silver salt solution and an aqueous halide solution to an aqueous protective colloid solution placed in a reaction vessel produces as many grain nuclei as possible at the time of nucleation in the initial stage of the addition. However, the continued addition of aqueous silver nitrate and halide solutions necessarily brings about the growth of the grain nuclei, so it is impossible in principle to obtain superfine grains which are extremely small in size (below 0.05 .mu.m).
On the other hand, JP-A-01-183417 (corresponding to U.S. Pat. No. 4,879,208) discloses a method of making silver halide grains, which comprises placing a mixing device outside a reaction vessel which contains an aqueous protective colloid solution and is designed to cause the crystal growth of silver halide grains, feeding aqueous water-soluble silver salt, water-soluble halide and protective colloid solutions into the mixing device and mixing these aqueous solutions therein to form fine grains of silver halide, and immediately thereafter feeding the fine grains into the reaction vessel to perform the crystal growth of silver halide grains in the reaction vessel. In the examples of the above-cited published patent application, grains expelled from the mixing device have a size below 0.05 .mu.m. That is to say, if nucleation is carried out in a mixing device and the grain nuclei are expelled from the mixing device as soon as they are formed, superfine grains extremely small in size can be obtained. However, the fine grains formed in the mixing device have very high solubility because of their fineness in size, so they cause so-called Ostwald ripening among themselves to result in an increase of grain size.
In other words, extremely fine grains having been once formed undergo Ostwald ripening during the washing, redispersion and redissolution steps, and an increase in grain size thereby results.
U.S. Pat. Nos. 3,661,592 and 3,704,130 disclose fine grains having grain sizes smaller than those of Lippmann emulsions (average grain size: 0.067 .mu.m), which are formed by adding an aqueous protective colloid solution and a grain-growth inhibitor to a reaction vessel, and then adding an aqueous silver salt solution and an aqueous halide solution thereto. In such a method, the prevention of an increase in grain size is intended by protecting against grain growth subsequent to nucleation in the reaction vessel. However, it is impossible to completely prevent grain growth in the reaction vessel by allowing such adsorbents as described above to adsorb to individual grain surfaces. The average grain sizes of the fine grains demonstrated in the examples in the specifications of the above-cited two patent were within the range of 0.05 to 0.03 .mu.m with respect to silver bromide.
Accordingly, fine grains smaller in size than Lippmann emulsions can be obtained, but it is still difficult to obtain superfine grains even smaller in size. Thus, the existing methods in the art have not made it feasible to make superfine grain emulsions having sizes far smaller than those of Lippmann emulsions, although such emulsions have been strongly desired.
Since fine grain emulsions prepared in accordance with the existing methods in the art are limited in the lower limit of their grain sizes, as described above, they are unable to ensure fully satisfactory properties for silver halide photographic materials containing them. Consequently, images recorded using those materials are insufficient in sharpness, which constitutes a very important factor in image quality, because of light-scattering and aggravation of granularity which are caused by the insufficiency in fineness of the silver halide grains.